Extendable struts as mechanical or kinematic members of larger machines are generally devices which are variable and controllable along one longitudinal axis, said axis originating at a fixed end and including an extendable end. Among various constructions, extendable struts can be built up of beam members slidably coupled to extend in one direction or along one longitudinal axis. Extension members which operate along more than one longitudinal axis are outside the scope of the invention.
Extendable struts are called upon to raise and lower loads, and may also be used to support a cantilevered load. Acting horizontally or at an angle reclined from vertical, adjustment of the extension can support the cantilevered close to or distant from the fixed end. One useful and commonly known example is raising and lowering a car in a repair shop for safe and easy access to components on its underside.
Besides locating a load or object by a specific, static adjustment of an extendable strut, dynamic extension and retraction of a strut can be used transport objects or move them relative to other objects or points of reference.
Whether driven by mechanically generated forces or by human muscle power, for extendable struts it is also important that the interfaces comprising slidable couplings also exhibit low friction. Many strategies exist for reducing friction at the interface between moving components, most crucially involving the prudent selection of materials sufficiently dissimilar to prevent galling or fretting corrosion in use.
Other strategies for reducing friction include depositing liquid lubricants or greases at the sliding interfaces, but these products attract dirt and dust, and retain tiny chips and wear particles shed by the load-bearing materials running against each other. Extraneous applied lubricants must be removed and regularly replaced when these become excessively contaminated with retained wear particles, or when they succumb to environmental or even biological deterioration such as when natural oils go rancid. The need for regular replacement necessitates additional tasks such as scheduled inspections or maintenance, and clean-out and disposal of waste lubricants. Cloying environmental sensitivities and stultifying regulations have made the handling and disposal of waste lubricants a tedious concern; increasing disposal costs which in turn accrue to maintenance costs and more complicated logistics. It is therefore an additional objective of the invention to eliminate or at least greatly reduce the need for lubricants to be applied to sliding interfaces of the invention.
Often, preferred positioning of an object supported by an extendable strut requires that rotation of the extendable end in the longitudinal axis, which is the axis of extension of the device, is constrained with respect to the fixed end. In the car repair shop lift example, it is unwanted that the car should rotate about the lifting axis while it is being raised or lowered.